Evaluation of Grape Maturity and the Factors Impacting Maturity

Transcription

1 Evaluation of Grape Maturity and the Factors Impacting Maturity Dr. B W Zoecklein Professor Emeritus and former Head of Enology-Grape Chemistry Group Virginia Tech Blacksburg Virginia bzoeckle@vt.edu Dr. B H Gump Professor of Beverage Management Florida International University Florida bgump@fiu.edu Definition of grape quality Berry development Vineyard factors impacting fruit maturation Climate Light Soil Water management and grape maturity Vine balance, yield and fruit maturity Asynchronous ripening Vine to vine variation Measuring vineyard variation Vineyard variation management Fruit Sorting Fruit sampling methods Fruit maturity gauges Berry size/weight Sugar evaluation 1

2 Sugar concentration and aroma/flavour Hang time - Potential for further ripening Berry shrivel and firmness Sugar per berry and sugar loading Brix-to-alcohol ratio ph, acidity and potassium Titratable acidity Organic acids Buffering capacity Phenolic compounds Factors impacting red wine colour Vineyard management and grape phenols Reductive strength Grape aroma/flavour and maturity evaluation Additional evaluations at harvest Nitrogen Glutathione Berry sensory analysis (BSA) Non-conventional maturity evaluation tools Grape sample processing Diseases and fruit rots Agrochemical residue Climate change and fruit maturity Conclusion References Definition of grape quality Science provides the knowledge to produce fruit that can result in consistent, flawless wines, but lacks signposts pointing the way to greatness. Minimalists correctly argue that fine wines have been successfully made long before our understanding of the science of grape growing and winemaking, and modern technology in general. This has added to the debate between the use of technology and non-interventionism. It also has highlighted the role of terroir. 2

3 Some suggest that a superior wine conveys a sense of place, originality and the natural telos of the site, a mantra echoed by virtually all premium wineries. This has spurred the interest in the concept of terroir in the New World. However, attempts to separate the kaleidoscope of variables associated with this term, including geology, geomorphology, soil, climate, the biology of the vine, microbiology and human interventions have proven difficult due to the complexity of interactions (van Leeuwen et al., 2013). Matthews (2015) suggests that today, terroir is primarily a marketing term that mixes extrinsic and intrinsic wine properties. Regardless, it is widely accepted that wine quality begins in the vineyard. High quality wines, regardless of how defined, are the result, in part, of the confluence of important fruit attributes. Grape quality is impacted by 1) maturity, purity, and condition, 2) aroma/flavour and phenolic characteristics, and 3) harvesting methods, transportation and processing protocols. Grape quality must be defined in terms of attributes suitable for a particular wine type and style. Physiological maturity is a term that is representational of the perceived temporal disconnect between aroma/flavour ripeness and sugar accumulation in the fruit. Ideally, desirable fruit components such as aroma/flavour and phenol components coincide with primary metabolites such as optimum soluble solids concentration. In reality, grape maturity indices seldom align; if they did, maturity evaluations would be an easy task. Berry Development There are three stages of berry development following flowering: green berry; arrest of green berry development, and the pause before the onset of ripening; and fruit ripening or véraison (Jackson and Lombard, 1993) (Fig. 4.1). Véraison can be divided into stages based upon berry metabolism and transport of substances to the vine (Fig. 4.1) (Bisson, 2001). Overall, the berry approximately doubles in size between véraison and harvest (Conde et al., 2007). As a result, many of the solutes accumulated in the fruit during the first period of development have their concentration substantially reduced. However, some compounds are reduced on a per-berry basis, not simply due to dilution. For example, malic acid, which is metabolized and used as an energy source during the ripening phase, is substantially decreased relative to tartaric acid, whose concentration usually remains almost constant after véraison. Tannins also decline significantly on a per-berry basis after véraison. Some aromatic 3

4 compounds, including several of the methoxypyrazine compounds, decline after véraison. Both changes in phloem transport and the onset of berry dehydration influence fruit composition (Matthews et al., 1990). Vineyard factors impacting fruit maturation A significant volume of research has advanced our understanding of how various viticultural variables and practices, including fruit maturity, crop level, crop exposure (Bergqvist et al., 2001; Zoecklein et al., 1998,1992), leaf area to crop ratio (Kliewer and Dokoozlian, 2005), shoot density and training systems (Reynolds et al., 1996) affect grape composition and maturation. Important features impacting fruit maturation beyond the general climate of the region and season include the following: Fruit temperature Humidity Soil characteristics Soil moisture Variety/clone Training and trellising systems Row orientation Canopy management Rootstock Yield components: fruit weight/vine, clusters per vine, clusters per shoot, berries per cluster, berry weight Climate It is well established that optimum wine quality requires the selection of the proper cultivar and clone on a desirable site. A clone is a population of plants, all of which are descendants by vegetative propagation from a single parent vine. Both cultivar and clonal selection can affect yield, fruit set, growth rate, clusters per vine, berry size, fruit rot susceptibility, and berry aroma/ flavour components. Field vine selection can be either by mass selection, where many vines are selected to provide bud wood, or by clonal selection, in which a single mother vine is selected to provide clones. Meteorological parameters have a crucial influence on fruit composition. The primary climate vectors impacting viticulture include temperature, moisture stress and radiation (Jones et al., 4

5 2012). It is well established that the phenology of bud break, flowering, and véraison are temperature dependent. Temperature affects the rate of fruit ripening. Sugar concentration increases with temperature, although secondary metabolites such as aroma/ flavour and phenol compounds are generally negatively affected by high temperatures (Kliewer and Torres, 1972). Climatologists recognize three levels of climate: macroclimate or regional climate, meso- or site climate, and micro- or grapevine canopy climate. Grapevine leaves are the major cause of microclimate variations; the presence of fruit, shoots, stems, and permanent vine parts are less significant (Smart, 1985). In the sense that grapevine canopy influences microclimate, it is under the control of the viticulturist. Canopy microclimate components include radiation, temperature, humidity and evaporation, each of which can impact fruit components. Berries maturing in densely shaded canopy interiors are generally associated with the following fruit attributes, when compared with berries in open or exposed canopies (Smart, 1985): low total soluble solids high titratable acidity high malate concentrations elevated ph high potassium low proline high arginine qualitative difference in tannin phenols low anthocyanin concentration in reds, and high chlorophyll versus flavonoid pigments in whites The above points to the importance of proper fruit sampling based upon the degree of cluster solar exposure (see below). The Huglin Index is similar to Winkler s macroclimate degree days, with the additional parameter of latitude on day length. However, Matese et al. (2012) have suggested commonly used climatic indices are not appropriate to represent vineyard meteorological variability, particularly the daily dynamics that are important in grape maturation. Molitor et al. (2014) 5

6 noted that the common cumulative degree day models used to forecast grape growth stages are often restricted to a limited number of phenological stages, or do not take into consideration the effects of higher temperatures. Jackson and Lombard (1993) divided grape-growing regions into two temperate zones: alpha zones, with mean temperatures of 9-15 C (48-59 F) during stage III, the final ripening period; and beta zones, with mean temperatures greater than 16 C (61 F). The best variety for any region is one that matches the length of the growing season, so that maturation occurs during the coolest portion of the season, allowing fruit maturity to occur just before the mean monthly temperature drops to 10 C/50 F. As such, studies have been conducted to adjust harvest dates within a region. For example, minimal pruning can delay berry ripening (Zheng et al., 2017). Palliotti et al. (2017) also demonstrated that double pruning can reduce berry sugar accumulation and its potential to delay harvest date or increase crop hang time under specific vineyard conditions. Palliotti et al. (2013) delayed sugar accumulation in Sangiovese by removing 30 35% of vine leaf area at Brix. By contrast, Zoecklein et al. (2011), using an ethanol spray, increased the ripening rate of Cabernet franc and Merlot. As suggested by Happ (1999), if the movement of temperature between the daily maximum and minimum exhibited the properties of a straight line, the mean would provide the average temperature experience. The true average lies away from the mean. The rise in a temperature curve is asymmetrical, and it changes with cloud cover, wind, etc. The optimum temperature for enzymatic reactions, which govern maturity, including aroma/flavour development and retention, is about 22 C. Therefore, it has been suggested that the periodic difference between the temperature experienced throughout the day (for example, every twenty minutes) and 22 C, is the true measure of site climate. As such, Happ (1999) calculated a heat load index, which takes into account the observation that a temperature rise does not necessarily have a linear effect on fruit components such as aroma/flavour. Table 4.1 illustrates how some viticultural and environmental factors can affect grape composition. Light Sunlight can affect grape maturation through photosynthetic and thermal responses. The amount of diffuse solar radiation reaching the interior canopy leaves and fruit decreases as the number of leaf layers increases (Smart, 1985), resulting in a reduction in photosynthetic rates. Varying shoot numbers, reducing vine vigor, or adopting training and trellising systems that 6

7 divide canopies into separate, thin curtains of foliage can influence grapevine microclimate and impact grape and resultant wine quality (Reynolds et al., 1996). Canopy microclimate can influence fruit maturation and quality via the following: heat light fruit rot incidence spray penetration relative humidity desiccation and reduction of evaporation potential Sunlight interception also depends on cloudiness and, to a much lesser degree, latitude (Gladstone, 2011). The sun is at a different angle on June 22 (longest day of the year) versus Fall equinox (September 23). At noon, when the sun is highest, row direction is less important. At 35 degrees latitude, in midsummer, north-south rows give about 17% more solar interception than east-west rows. In the northern hemisphere, the sun is in the southern part of the sky for most of the day during the summer. Fruit on the southern side of east-west rows will receive more light. North-south rows may have a disadvantage in warm climates where fruit on west sides goes from morning shade to direct sun exposure at mid-day (Gladstones, 1992). As such, differences in row orientation and canopy side can impact fruit maturation rate and fruit volatiles (Zoecklein et al., 2011; Devarajan et al., 2011), suggesting the importance of differential harvest dates. Soil Soil is a complex medium and its role involves the multiple influences of texture, mineral composition, water supply, and root zone temperature, among other variables (van Leeuwen, 2013). Jackson and Lombard (1993) reported that soil is known to have several direct influences on plant growth by affecting moisture retention, nutrient availability, heat and light reflecting capacity, and root and vegetative growth. Duteau et al. (1981) found that soil and its geological composition, not microclimate, was the major factor influencing grape maturation in St. Emillion. According to Barbeau et al. (1998), early grape maturing sites are characterized by sandy, sandy-clayey, or gravelly soils with good drainage. Late maturing sites often have clayey or silty soils. Soil Information Systems are available to evaluate detailed soil properties for each block, such as compaction, root zone depth, moisture retention and fertility. 7

8 Integrated pest management (IPM) is now the common practice of promoting the natural ecological balance of flora and fauna in the vineyard. Such practices are now frequently used as alternatives to heavy tillage, pesticides and herbicides to create what is referred to as living soils, that is, with a healthy earthworm population. Such soils are reportedly associated with wines that provide enhanced reductive strength (Smith, 2013). The term microbial terroir has been adopted to relate to soil ecology, specifically the microbiology of the soil. Although a good soil should have adequate microbiological flora to aid in mineralization, little scientific evidence is available to suggest the link between soil microbes and wine quality or terroir (van Leeuwen et al., 2013). It is interesting to note the level of copper (in the form of Bordeaux mix) used in some French vineyard sprays (up to 3 kg/ha/year). Such sprays over many years may have had a detrimental impact on soil microorganisms (Courde et al., 1998). Yeasts and bacteria are part of a complex series of interactions where competition, equilibrium and collaboration form a dynamic ecosystem. Even with the addition of sulfur dioxide and cultured yeasts to a red must, a portion of a fermentation can be conducted by other, native uninoculated organisms (Bokulich et al., 2012). There can be a substantial difference in microbial populations among different vineyards and that microbial ecology can be a source of wine variation. Some winemakers report that certain vineyards produce wines that are more prone to Brettanomyces spp. growth than others. The implication is that this spoilage yeast is coming from the vineyard. Mansfield et al. (2002) and Fugelsang and Zoecklein (2003) demonstrated the significance of regionality among Brettanomyces spp. strains, which may help explain this observation. Water management and grape maturity Some winemakers believe that dry farming (the absence of irrigation) produces fruit and wines that more fully express the nature of the vineyard site, at least in arid regions. Some European vignerons equate limited soil moisture with their terroir expression and remain reluctant to irrigate, even when legally permitted. Catena (2016) suggests that many of these vineyards are on abundant underground aquifers. In many other regions of the world, aquifers are very deep, and thus the water is unavailable to vines. 8

9 Vine water status depends on soil texture, percentage of stones, rooting depth, rainfall, evapotranspiration and leaf area (van Leeuwen and Darriet, 2016). Basile et al. (2011) noted that berry composition significantly correlated with the vine water status, but the nature of the relationship depended on the phenological stage and the parameter measured. Water deficiency affects photosynthesis and shoot growth, and can increase both tannin and anthocyanin content (Duteau et al., 1981), while excess stress can lead to leaf damage and severally impair fruit ripening. Chapman et al. (2005) found that vine water deficits lead to wines with more fruity and less vegetal aromas and flavours than vines with high vine water status. It has also been reported that irrigation practices such as regulated deficit irrigation (RDI) positively impact the fruity aromas (Casassa et al., 2013; Gamero et al., 2014). Keller et al. (2016) demonstrated that the potential effects of water deficit on fruit composition may be related to altered canopy size and microclimate, in addition to decreased berry size. Roby et al. (2004) demonstrated that there are effects of vine water status on fruit composition that arise independently of the resultant differences in fruit size. In their study, the effect of vine water status on the concentration of skin tannin and anthocyanin was greater than the effect of fruit size on those same variables. Pre-véraison water deficit can accelerate fruit pigmentation and colour change earlier than non-water deficit fruit (Herrera et al., 2016). The proper cover crop may help assure ground shading and contribute to humus formation while helping to buffer the very dry and very wet periods. As such, vineyard floor management has multiple goals that encompass improving weed management and soil conservation, reducing soil resource availability to control vine vigor, and influencing fruit and wine quality. In one study, Guerra and Steenwerth (2012) reported that cover crops increased juice soluble solids, anthocyanins, and other phenolic components, and decreased ph. Fruit ripening is influenced by plant hormones. Optimum hormone balance is dependent on a continuous and moderate moisture stress and favorable soil temperatures (Gladstone, 2011). Water stress, through the stimulation of stress hormones such as abscisic acid and the suppression of growth hormones such as gibberellins, cytokyinins and auxins, stimulate the production of enzymes, promoting flavour ripening in the fruit. The majority of these changes occur just prior to and during véraison. As such, managing mild water stress at this time may allow for optimum aroma/flavour peaks, possibly prior to excess sugar production (Greenspan, 2018). 9

10 Vine balance, yield and fruit maturity Fruit ripening is dependent upon source leaves, a reason for the general interest in the concept of vine balance and an understanding that high yield does not necessarily mean low quality (Matthews, 2015). For fruit at similar maturity, factors other than yield, such as water availability, may determine fruit composition (Matthews, 2015). The many components contributing to grapevine yield include the following (May, 1972): Vines per acre/vines per hectare Shoots per vine/shoots per meter Clusters per shoot Clusters per vine Cluster weight Berries per cluster Berry weight Fruit weight per vine Yield can impact the rate of fruit maturation (Winkler, 1965). An over-cropped vine is one that has a large crop with insufficient, healthy active leaves; it cannot produce enough sugar to maintain all clusters for desirable ripening, and it fails to produce grapes with sufficient aroma/flavour and/or desirable phenol compounds. Variation in components of yield can contribute to yield variation at harvest, although the grapevine itself is capable of self-regulation (Clingeleffer, 1983) and yield compensation (Freeman et al., 1979; Smart et al., 1982). While many yield components cannot be controlled directly, vineyard managers do have the capacity to manipulate some variables in the vineyard. For example, pruning regulates node number per vine and budburst. Vine balance is known to impact plant hormone concentrations. Higher levels of the growth hormone cytokinin are stimulated by high nutrient and water availability. This excess of cytokinin may dominate the mainly leaf-borne ripening hormone abscisic acid and, thus, delay ripening and aroma formation. A vine under nutrient and/or water stress has a dominance of abscisic acid over cytokinin, resulting in a hastened ripening rate (Gladstones, 2011). 10

11 Berry weights can be used to estimate crop load. There is a relationship between berry weight at véraison and berry weight at maturity. For Syrah, McCarthy (1997) determined that relationship to be the following: y = 1.35x , where y = the berry weight at 23 Brix, and x = the berry weight at about 5 Brix. This relationship will differ by cultivar and site, but can be determined by collecting véraison and harvest samples for several seasons. Accurate estimations of yield from precision viticulture techniques, with mapping using GPS systems, optical remote sensing and other tools, are available. Asynchronous ripening and measuring vineyard variation Variation in the vineyard occurs among berries, bunches, and vines and can have a negative impact on crop level, fruit composition, and wine quality. Two components of berry-to-berry variation are size and berry composition. In extreme cases, this is referred to as hen and chicken or millerandage (Winkler, 1965). Variation in berry size affects vineyard yield and may impact wine quality. However, Matthews (2015) reported that smaller berries obtained by water deficits had an increased colour and tannin concentration. Smaller berries resulting from canopy shade had the opposite effect. High levels of variation in the early post-flowering period suggest that that variation originated prior to berry set, likely as a result of asynchronous cell division in the floral primordium at budburst. Decreasing levels of variation may indicate points of resynchronization in the berry growth cycle. A crop with asynchronous clusters or berries has a mixture of developmental stages, resulting in berries with optimal qualities diluted by berries which may be inferior. The practical significance of this dilution depends upon the degree and stylistic goals. There are those who believe some asynchrony aids in complexity. Figure 4.2 demonstrates a frequency distribution, with berry numbers plotted against Brix. Even before differences arise from processing, it is generally not true that two vineyards or vineyard blocks with the same Brix values will give similar wines. A juice with Brix of 22 might be composed of a narrow distribution of a few berries at 20 and a few at 24 Brix, with the majority nearer to 22. However, there may be a much wider distribution, with berries below 18 and greater than 24. Because Brix is a distribution average, juices with similar Brix values can produce quite different wines, due to variations in aroma/flavour and phenol compounds. Relative maturity dates of the various important components of a red berry (skin, pulp, seeds, and cap stem) are generally different. Given that all parts enter the fermenter in red wine production, the control of stylistic winemaking may be negatively influenced if component parts of the fruit are not at the 11

12 optimal maturity at harvest. The variables that contribute to variation among berries include berry size, berry composition, seed number, seed size, degree of lignification etc. Vine to vine variation Many variables can be measured at the vine level, including soil characteristics, carbohydrate reserves, bud fruitfulness, percent budburst, inflorescence primordia number, node number, shoot number and cluster number. Vine-to-vine variability of visually uniform vines, expressed as percentage of the coefficient of variation, was reported by Gray (2006), highlighting the inherent nature of vineyard variability. While soluble solids concentrations may be fairly uniform, with a coefficient of variation usually less than ten per cent, the variance can be much greater if the fruit is not uniform across clusters or if the cluster microenvironment is variable among vines: Brix 4 to 5% ph 3 to 4% Titratable acidity 10 to 12% Berry weight 6 to 20% Colour 13 to 18% The inherent variation among individual vines can have a greater impact on yield than external influences such as soil variability, or drainage and fertility irregularities. Variation in soluble solids concentrations, titratable acidity, and cluster weight between vines can be much greater than within vines (Rankine et al., 1962). Spatial analysis techniques and global positioning systems (GPS) have aided our understanding of vineyard variability. Aerial vineyard images, using satellite or aircraft, can be used to calculate a normalized difference vegetation index (NDVI) for each vine. These maps can be used to visualize differences in vine vigour or relative biomass on a vineyard scale (Hall et al., 2002), which may allow for differential harvests. Differences in cluster size are commonplace in most vineyards. Since yield forecasting and maturity testing procedures may rely on cluster sampling, differences in cluster size can be a major source of error. Stratified cluster and berry sampling programs have been devised to overcome some of these problems, but seasonal, varietal and site-specific considerations confound general sampling protocols (Wolpert and Howell, 1984; Kasimatis and Vilas, 1985). The variables that contribute to variation among bunches 12

13 include inflourescence primordia size, flower number, fruit set, berry number, cluster weight, and cluster position. Measuring vineyard variation A number of studies have reviewed the factors impacting vineyard variation (Rankine et al, 1962; Smart and Robinson, 1991; Trought, 1996; Trought and Tannock, 1996). Prior to fruit sampling, one needs to gain some appreciation of the variation within each vineyard block that can be influenced by microclimate effects, which can result in differences in heat, light and soil moisture. Several techniques can be used to quantify the level of dispersion around a population mean, including range, mean deviation, sum of squares, variance, standard deviation, and coefficient of variation. Expressed as a percentage, the coefficient of variation (CV) is a unitless measure of the sample variability, relative to the sample mean: coefficient of variation (CV) = standard deviation (s) x 100 mean (x) A sequential comparison of CVs can reveal both the source of variation and the points of re-synchronization in the berry s developmental cycle (Gray, 2006). Vineyard variation management Zonal management and zonal harvest are appropriate techniques where the grape grower has ready access to the necessary technology. Perhaps the best approach to help minimize vine variation is site selection. Variation may be minimized by choosing a site with limited variation in soil, topography, aspect, and extreme weather events, optimally suited for the variety. Cluster variation may be managed by applying viticultural best practices or a viticultural Hazard Analysis and Critical Control Point (HACCP) plan to promote uniform bud burst, shoot growth, flowering, cluster exposure and berry development (Coombe and Iland, 2004). Factors that may contribute to variations include cluster architecture, the role of vascular function in berry growth and development, the relationships between seed development and berry development, and the relative importance of cell division and cell expansion throughout the entire developmental cycle (Gray, 2006). 13

14 Fruit Sorting Manual sorting in the field is generally supplemented by additional sorting practices in the winery. Optical sorters performing selection based on size, colour, and level of berry shrivel are available which aid in stylistic winemaking. Being able to sort high Brix and high-coloured red fruit, for example, from average colour and Brix, and being able to separate additional material other than grapes (MOG) and fungal degraded berries also adds additional quality and stylistic freedom. Ward et al. (2015) highlighted the importance of sorting by demonstrating that the concentrations of the predominant methoxypyrazine in the wines, 3-isobutyl-2-methoxypyrazine, increased with increasing additions of unripe berries to the must. Research into hyperspectral sorting based on compounds visible in the UV range, such as organic compounds, will likely increase and may result in significant increases in wine quality and vintage uniformity. Fruit sampling methods Regardless of maturity gauges utilized, an important concern is accurate vineyard sampling. Fruit sampling methodologies have been extensively reviewed (Rankine et al., 1962; Roessler and Amerine, 1963; Jordan and Crosser, 1983; Kasimatis and Vilas, 1985; Wolpert and Howell, 1984; Gray, 2006). There are two basic choices in fruit sampling: cluster sampling or berry sampling. With cluster sampling, a further choice can be made by gathering clusters from throughout the vineyard, or using one or more targeted vines. If berry sampling is to be employed, two samples of 100 berries each can give accuracy to 1.0 Brix, and five samples of 100 berries each can give accuracy to 0.5 Brix. Using cluster sampling, ten clusters can be accurate to 1.0 Brix (Jordan and Croser, 1983; Kasimatis and Vilas, 1985). The three factors which have a major role in maturation dynamics are heat, light and soil moisture. Therefore, variation of these within a vineyard block can result in significant sample variation. It should be noted that there is a general tendency, when examining a cluster prior to berry sampling, to select the most mature berries. Therefore, berry sampling should involve locating the fruit zone, and sampling without examining the clusters or berries. Samples should be collected from the top, middle and bottom of the cluster while randomizing the side of the cluster sampled. If this does not occur, berry samples will frequently be about 2 Brix higher than the true value. About 90% of the variation in berry sampling is believed to come from variation in the position of the cluster on the vine and the degree of sun exposure (Jordan and Croser, 1983). Therefore, 14

15 vineyards must be sampled based on the degree of fruit exposure using the following protocol: Avoid edge rows and the first two vines in a row, and collect samples from both sides of the vine. For each row, estimate the proportion of shaded bunches and sample accordingly. Maximum sample area should be less than 2 hectares. Applying traditional statistical models to vineyards with known field variability can lead to inefficient sampling. Meyers and Vanden Heuvel (2014) used aerial normalized difference vegetation index (NDVI) images for the purposes of quantifying vineyard spatial structure and computing optimal vineyard sampling protocols. Bramley (2005) suggested that in the absence of zonal management, a winemaker s ability to maximize benefits from differential vineyard management, such as selective harvesting, is unlikely to be satisfied. Fruit maturity gauges Maturity evaluation must be viewed in the context of stylistic goals. Maturity evaluations usually involve a review ofseveral to many of the following (Zoecklein et al., 1999): Aroma/flavour, and intensity of aroma/flavour Grape skin tannins and tannin extractability Red fruit colour/anthocyanins Stem lignification or ripeness Seed numbers per berry Seed ripeness, maturity or tannin extractability Sugar per berry Brix Acidity ph Berry softness Berry size/weight Berry shrivel Potential for further ripening, general fruit condition It is not completely understood how each of the above relate to one another, or the importance of their individual or collective values as predictors of ultimate wine quality. The time to harvest is prior to deterioration of desirable fruit characters or components. While desirable attributes do 15

16 change over time, grapes change physiologically rather slowly at the end of the season, less the impact of fungal outbreaks and detrimental weather (Matthews, 2015). However, the factors that control the loss of berry aroma/flavour compounds, for example, and when degradation may be initiated, is not well understood. As such, a chemical marker of the onset of fruit aroma/flavour deterioration would be ideal as a maturity gauge, as suggested by Bisson (2001). Berry size/weight Many winemakers determine berry size via weight. Many believe that smaller berries may yield richer must, in terms of colour intensity and tannin composition. However, Matthews and Kriedemann (2006) reported that the cause of berry size is more important in determining must composition and wine sensory properties than berry size per se. They suggested that how the change in size came about is important, making a distinction between environmental factors versus biological processes that underlie variation in reproductive development. For example, smaller berry size in red varieties, such as Cabernet franc, commonly yields a richer must if berry size is reduced by environmental factors such as deficient irrigation. By contrast, Shiraz berries that are smaller for developmental reasons, and have fewer seeds, do not necessarily produce musts that are richer (Walker et al., 2005). High total yield reduces the weight of individual fruit, but generally causes lower, rather than higher, concentrations of solutes (Bravdo et al., 1985). Increased light exposure increases both berry size and solute concentration (Dokoozlian and Kliewer, 1996). Additionally, the timing of water deficits prior to véraison often, but not always, increases Brix. The question remains whether that is solely the consequence of a reduction in berry size. Knowledge of berry size may allow for adjustments in wine processing methodologies such as cap management and saignée to reach stylistic goals. Sugar evaluation Sugar is usually expressed as Brix or total soluble solids concentration (TSS), Baumé or potential alcohol, or by specific gravity. Brix is defined as grams of soluble solids per 100 g of solution. It is a measure of all soluble solids, including pigments, acids, glycerol, and sugar. Generally, the fermentable sugar concentration of grape must accounts for 90 to 95% of the total soluble solids. Therefore, determination of Brix provides only an approximate measurement of sugar concentration. The vast majority of grape sugar consists of the two monosaccharides glucose and fructose. The ratio of these two is dependent upon the variety 16

17 and the extent of fruit maturity, with glucose dominating during early berry development. Overripe fruit generally has a low glucose-to-fructose ratio, which can have implications with regard to fermentation completion (Zoecklein et al., 1999). Baumé, often used in Europe and Australia, is an estimation of the potential alcohol, a measure of the sugar concentration of fruit and the potential alcohol that can be achieved by complete fermentation. Thus, Brix and Baumé naturally relate to each other: 1.0 Baumé is equivalent to 1.8 Brix. Grapes with a 13 Baumé, if fermented completely, would produce a wine with about 13% (v/v) alcohol. Sugar concentration and aroma/flavour A number of studies have shown a correlation between sugar accumulation and grape berry aroma/flavour compounds, however, the strength of the association depends on a number of variables (Robinson and Davies, 2000). The synthesis of many grape aroma/flavour compounds requires energy, but the factors leading to cessation of synthesis have not been well defined. In cold to cool heat summation regions, Brix is generally more strongly correlated to aroma/flavour than in warmer regions (Jackson and Lombard, 1993). Strauss et al. (1987) demonstrated that one group of aroma/flavour compounds, norisoprenoids, are strongly correlated to grape sugar. Norisoprenoids, 13-carbon terpenoids, are derived from the degradation of carotenoids, and are associated with descriptors such as grassy, tobacco, smoky, kerosene, tea and honey (Strauss et al., 1987). The norisoprenoids appear to be more stable than the compounds associated with fruity aroma notes. Boss et al. (2014) demonstrated the complex relationships among sugar content, harvest date, and wine volatile composition. They reported that monoterpenes generally increased in abundance in relation to increasing Brix, with less of an effect due to harvest date. Compounds that decreased in abundance in relation to Brix were also influenced by harvest date. Many of these compounds were acetate esters of higher alcohols, as well as ketones and acetals. The positive impact volatiles accumulate is closely related to increasing Brix. However, the loss of compounds that may impart negative attributes may be a passive process and require a certain amount of time on the vine. The main compounds responsible for green aromas in grapes and wines are 3-isobutyl-2-methoxypyrazine and C 6 compounds. Mendez-Costabel et al. (2013) found that seasonal variation was more important than regional variation, and similar trends among regions were found within each season. Temperature during the spring, a period of 17

18 active growth, was found to be a significant driver of fruit green aroma compounds at harvest, likely due to its interactions with vine vigor and fruit shading. Thus, while sugar can indicate general maturity level, it is not a clear estimation of aroma/flavour. Hang time - Potential for further ripening The limited correlation between changes in Brix and aroma/flavour and phenol compounds has resulted in the concept of physiological maturity and the expression hang time. A typical sugar profile during ripening shows an initial rapid accumulation, but at some point during development, the vine ceases transport of sugar to the fruit. Sugar accumulation occurs only to a certain point, usually around 24 Brix; further increases in sugar concentration are due to dehydration. Brix, berry aroma/flavour, and phenol maturity are not always strongly correlated. This has resulted in extended fruit hang time to allow for desirable changes in secondary metabolites. The results may include the loss of fruit weight, increases in Brix (often as a result of dehydration) and the elevated level of potential wine alcohol. Luna et al. (2017) reported that delayed harvest date had a greater effect than crop reduction on fruit composition. Figure 4.3 illustrates the relationship between berry weight and Brix at several sampling dates. As maturation continues, berry weight increases, then declines. This decline frequently occurs prior to harvest. Brix can increase in late stages of maturity, either due to the production of sugar by the plant, or to dehydration of the berry. Mathews (2015) suggests that the so-called Old World style is associated with a lower Brix with wines categorized as having finesse. This is contrasted by New World wines, notably in warm climates, frequently harvested at higher Brix levels and sometimes called fruit bombs. Both New World and Old World wines have merit, which suggests the limits of the association of quality to concentration. Berry shrivel and firmness Grape maturation can be evaluated by assessing physical properties of the berry, such as firmness and deformability. Berry softening is due to changes in composition of cell walls of the fruit, particularly due to pectin and xyloglucan depolymerization, which accompanies arrest of xylem flow to the fruit (Rogiers et al., 2006). This softening can result in increased skin tannin extraction at crush, resulting in a form of fining caused by precipitation of the larger molecular weight astringent tannins with cellular components (Keller 2011). Thus, changes in the tannin distribution in the juice and subsequent wine can occur. 18

19 Berry shrivel is an important attribute impacting yield and, frequently, wine style. Shrivel is particularly notable in some varieties such as Shiraz, where shrinkage begins in warm regions at about 80 to 90 days post-flowering (McCarthy and Coombe, 2001). The decline in berry weight is more closely related to the time from flowering than to Brix. Symptoms include loss of berry turgidity and wrinkling of the skin. The rate of berry shrinkage varies as a result of region, season and/or climatic conditions, and among vines within blocks (Rogiers et al., 2006). Between the maximum berry weight and time of harvest, there can be substantial decline in weight. In one study, McCarthy and Coombe (2001) determined optimum harvest weight for maximum secondary metabolite concentration in an Australian Syrah to be 1.2 g per berry. The incidence of berry shrivel and degree of shrivel is used as a maturity gauge for some varieties. Sugar per berry and sugar loading The Brix of grape must accounts for 90 to 95% of the fermentable sugars. However, this measurement is a ratio (wt/wt) of sugar to water and may change due to physiological conditions in the fruit. A potential problem encountered in Brix, Baumé, or any soluble solids measures used as a fruit maturity index, occurs with changes in fruit weight. Over time, soluble solids readings may show no change but, in fact, there may be substantial changes in the fruit weight, either increases or decreases (Table 4.2). Sugar accumulation may cease due to unfavourable environmental conditions, such as very high or low vineyard temperatures, but resume once conditions have changed. It is important to be able to distinguish transient effects from the permanent cessation of transport of photosynthates. Once phloem transport has ended, any further increases in Brix will be due to loss of water, not continued synthesis and translocation of sugar. Assessing changes in berry weight, and noting the point at which average berry weight starts to decrease while Brix increases, can indicate the onset of dehydration. However, this can be difficult to monitor where fruit maturity is not uniform across clusters or berries. The concept of sugar per berry utilizes a soluble solids evaluation such as Brix, and takes into account the weight of a berry sample. For example, if data were taken from the same vineyard at 5-day intervals and the soluble solids ( Brix) of both sample dates measured 22 Brix, it might be concluded that there had been no change in fruit maturity. However, sugar per berry calculations could lead to a different conclusion if there were changes in berry weight (Table 19

20 4.2). Sugar per berry calculations yield considerably more information than that available by evaluation of Brix measurements alone. Research indicates that the maximum rate of production of aroma/flavour compounds occurs at about the time the berry stops importing water from the phloem, or shortly thereafter. Therefore, maximum aroma/flavour occurs sometime after the berry reaches maximum weight in most instances, suggesting the importance of this as a stylistic winemaking tool. Some industry practitioners use sugar loading peaks (increase of less than 3 mg/berry/day) as a method of maturity evaluation. By measuring when the vine has stopped exporting sugar into the fruit, harvest can be conducted at intervals thereafter. Time spans of 5-7 days post sugar maximum can result in qualitative differences in both the aromatic and mouth-feel features highlighted in the resultant wines. Brix-to-alcohol ratio Producing balanced harmonious wines is an important industry goal. Balance refers to the relative concentrations of volatile and structural/textural components (Zoecklein, 2013). Making wine in a warm growing region or vineyard site may pose a challenge with regard to avoiding excessive alcohol concentration, where increased hang time can result in alcohol levels that are relatively high and negatively impact wine balance. Theoretically, a given weight of fermentable sugar will yield 51.1% alcohol by weight. The actual alcohol yield is generally different from the theoretical. In the past, winemakers used the conversion factor of 0.55 multiplied by the Brix to estimate the potential alcohol produced in a dry wine. However, the actual conversion rate can vary from 0.54 to 0.62, or higher. These differences are the result of several factors listed below. For example, softening of grapes occurs from véraison to harvest as a result of changes in pectin polysaccharides. Increases in deformability occur with increases in the water-soluble polysaccharide concentration, which can increase the non-sugar soluble solids concentration. The soluble solids to alcohol ratio can be influenced by the following: Variety Season Maturity level/soluble solids Fermentation temperature 20

21 dioxide) Open vs. closed fermenters (alcohol loss due to entrainment with carbon In many regions the alcohol levels of the resultant wines are higher than desired for optimum balance. As such, where legally permissible, some choose to water down, that is add water to the juice per-fermentation to reduce the alcohol potential. ph, acidity and potassium Assessments of acidity and ph are used to help define the optimal time of harvest for a particular wine style. Both are known to have significant impacts on wine (Zoecklein et al., 1999). The ph values for white wines may be 3.5 or less. Higher values are usually observed for red wines, largely because of contact of juice and skins before and during fermentation. Changes in fruit ph are complex and the result of a number of environmental and viticulture management factors. Grapes are rich in potassium, an essential macronutrient for growth and development. Potassium ion (K + ) is the main cation in must and wine (Blouin and Cruège, 2003) and is absorbed by the roots and distributed to all parts of the vine. Early in the season, when the growth rate is high, much of the K + accumulates in the leaves. After véraison, a sharp increase in berry K + is observed as a result of K + redistribution from leaves to berries (Ollat and Gaudillère, 1996; Blouin and Cruège, 2003). Excessive K + concentration in the fruit at harvest may result in increases in ph and thus negatively impact potential wine quality, particularly in red wines (Davies et al., 2006). The stoichiometric exchange of tartaric acid protons with K + cations results in the formation of largely-insoluble potassium bitartrate, leading to a decrease in free acid and tartrate-to-malate ratio (Gawel et al., 2000). The overall result is an increase in ph. High K + levels in the berry may decrease the rate of malate degradation by impairing malate transport from the storage pools in the vacuole to the cytoplasm. Grape skin contains from three to 15 times more K + than is present in the pulp. Therefore, berry K + levels are often more important to red than to white wines, due to skin contact in red wine production (Mpelasoka et al., 2003). The levels of K + in grape berries may be affected by numerous factors, including K + level in the soil, antagonistic elements in the soil such as magnesium and calcium, grape variety, soil moisture and viticultural practices (Mpelasoka et al., 2003; Davies et al., 2006). Several vineyard management considerations impact K + uptake and ph evolution. Severe stress late in the season can increase K + uptake. Potassium concentrations have a significant impact on juice and wine 21

22 buffering capacity (see below). Crop and overall vine balance are also important in helping to manage ph evolution. Over-cropping may delay the rate of fruit maturity, which can result in increases in ph. Titratable acidity The acid concentration of fruit and resultant wine is important to structural/textural balance. Titratable acidity (TA) in grapes normally ranges between 5.0 and 16.0 g/l expressed as tartaric acid; these values are influenced by variety, climatic conditions, cultural practices, and maturity of the fruit. The reduction in TA during fruit ripening is partly related to the respiration of malic acid in the berry and is, therefore, related to temperature. Grapes grown in warmer regions (more heat summation units) mature earlier and have a lower TA at the same soluble solids concentration, when compared to fruit grown in a cooler climate (Gladstones, 1992). A characteristic of cooler growing regions is lower daily temperature fluctuations during the late stages of fruit ripening, an important contributor to acid retention (Gladstones, 1992). Organic acids Malate is consumed as an energy source in the berry during véraison, and the concentrations decrease relative to tartrate (Jackson and Lombard, 1993). Tartrate concentrations generally remain constant during véraison, but may rise slightly during grape dehydration. Malate concentrations decrease with maturity, and may plateau at a low level, roughly 2 to 3 g/l (Jackson and Lombard, 1993). Grapes may catabolise sugar if malate concentrations decline too much, depending upon the variety (Conde et al., 2007). Generally, the malate-to-tartrate ratio does not appear to correlate well with aroma/flavour production in the fruit. Generally, the warmer the season, the lower the malic acid content. There is a strong correlation between the malic acid concentration and the concentration of an important group of grape-derived aroma compounds, the methoxypyrazines. Methoxypyrazines such as IBMP (2-methoxy-3-isobutylpyrazine), a nitrogen-containing plant metabolite, can impart a vegetal aroma to some varieties including Cabernet Sauvignon, Cabernet Franc, and Sauvignon Blanc. Described as bell- or green pepper-like, excessive concentrations of IBMP can negatively impact the aromatic quality of wines. Decreases in pyrazines are the result of fruit maturation and temperature (Allen, 2006). The decrease in IBMP is directly correlated to malic acid decline (Roujou de Boubee, et al., 2000). 22

23 Buffering capacity The buffering capacity of juice or wine is a measure of its resistance to ph changes. Buffering capacity is particularly important in regions and seasons where fruit ph at harvest may be elevated and the winemaker desires to acidulate to lower it. Essentially, buffering capacity is a measure of the organic acid pool (malic and tartaric) at winemaking ph. A system with a high buffering capacity requires more hydroxide (OH - ) ions or hydrogen ions (H + ) to change the ph than one of lower buffering capacity. Thus, buffering capacity can be defined in practical terms as the quantity of hydroxide or hydrogen ions needed to obtain a change of one ph unit (e.g., from ph 3.4 to 4.4). The net result of buffering action is to create, within the system, resistance to changes in ph that otherwise would occur with addition of either acid or base. In the case of base addition, excess OH - ions are consumed by H + ions of the buffer s acid component to form water, whereas excess protons are consumed by the anion component. Generally, the higher the fruit K + level, the greater is the buffering capacity. Buffering capacity explains why each year is unique in the relationship between acidulation and ph changes, since the buffering capacity of fruit changes as a function of growing conditions and subsequent fruit chemistry. A method for determination of buffering capacity is described by Zoecklein et al. (1999). Phenolic compounds The quantitative and qualitative evaluation of fruit phenolic compounds is used as important maturity gauges. In general, higher phenolics are associated with higher sunlight exposure, lower nitrogen levels, lower soil moisture, moderate canopy size, moderate crop load, lower soil fertility and smaller berry size (Kennedy 2018). Classes of compounds of particular importance to winemakers include: hydroxycinnamates, tannins, and the flavonoid phenols anthocyanins, and flavonols. Hydroxycinnamates are found mainly in the pulp and can be converted to unpleasant aroma compounds by spoilage yeasts, including Brettanomyces sp. Over exposure of fruit to sunlight elevates their production. The term tannin defines a heterogeneous group that are identified based on certain properties: astringency bitterness reaction with ferric chloride ability to bind with proteins, 23